The applications of SnO2-SWNT (tin oxide-single-walled carbon nanotubes) hybrid structure as room temperature gas sensing platform has been demonstrated in previous studies. Combining efficient transduction property of SWNTs with high molecular detection property of SnO2, excellent sensitivities to trace quantities of both oxidizing (limit of detection (LOD) of 25 ppb for NO2) and reducing gases (LOD of 10 ppm for H2) at room temperature was observed. The enhanced sensing performance observed for these hybrid nanostructures compared to unfunctionalized carboxylated SWNTs, is attributed to the availability of increased surface area of active elements, which can take part in gas molecule interactions. Although promising results were observed for these hybrid nanostructures, further improvement in sensitivity and particularly selectivity towards specific analytes remains a challenge.
Typically, sensors using SnO2 as a sensory element use small amounts of additives such as Pd, Pt, Au, Ag, etc., to increase sensitivity and selectivity towards specific analytes. Generally, two different mechanisms have been considered to explain the observed enhancement in sensing performance for metal particles impregnated tin oxide sensors. The first is called chemical sensitization, where the metal particles catalytically activate the redox processes occurring at the tin oxide surfaces by lowering the activation energy for dissociation of analyte gases such as O2, H2, H2S, CO, etc. The activated products then migrate towards the tin oxide surface, to react with adsorbed oxygen species resulting in a greater and faster degree of charge transfer between tin oxide and the adsorbate. The second mechanism is called electronic sensitization, where the metal nanoparticles interact electronically with the tin oxide surface forming charge depletion zones around the particles. Any changes observed in the work function of the additive due to gas adsorption and desorption will cause a change in the Schottky barrier between the metal particle and tin oxide resulting in conductivity changes. The two processes are schematically represented in FIG. 1.